The speed of a hydraulically driven working member on a machine depends upon the cross-sectional area of principal narrowed orifices of the hydraulic system and the pressure drop across those orifices. To facilitate control, pressure compensating hydraulic control systems have been designed to set and maintain the pressure drop. These previous control systems include sense lines which transmit the pressure at the valve workports to the input of a variable displacement hydraulic pump which supplies pressurized hydraulic fluid in the system. The resulting self-adjustment of the pump output provides an approximately constant pressure drop across a control orifice whose cross-sectional area can be controlled by the machine operator. This facilitates control because, with the pressure drop held constant, the speed of movement of the working member is determined only by the cross sectional area of the orifice. One such system is disclosed in U.S. Pat. No. 4,693,272 entitled "Post Pressure Compensated Unitary Hydraulic Valve", the disclosure of which is incorporated herein by reference.
Because the control valves and hydraulic pump in such a system normally are not immediately adjacent to each other, the changing load pressure information must be transmitted to the remote pump input through hoses or other conduits which can be relatively long. Some hydraulic fluid tends to drain out of these conduits while the machine is in a stopped, neutral state. When the operator again calls for motion, these conduits must refill before the pressure compensation system can be fully effective. Due to the length of these conduits, the response of the pump may lag, and a slight dipping of the loads can occur, which characteristics may be referred to as the "lag time" and "start-up dipping" problems.
In some types of hydraulic systems, the "bottoming out" of a piston driving a load could cause the entire system to "hang up". This could occur in such systems which used the greatest of the workport pressures to motivate the pressure compensation system. In that case, the bottomed out load has the greatest workport pressure and the pump is unable to provide a greater pressure; thus there would no longer be a pressure drop across the control orifice. As a remedy, such systems may include a pressure relief valve in a load sensing circuit of the hydraulic control system. In the bottomed out situation, the relief valve opens to drop the sensed pressure to the load sense relief pressure, enabling the pump to provide a pressure drop across the control orifice.
While this solution is effective, it may have an undesirable side effect in systems which use a pressure compensating check valve as part of the means of holding substantially constant the pressure drop across the control orifice. The pressure relief valve could open even when no piston was bottomed out if a workport pressure exceeded the set-point of the load sense relief valve. In that case, some fluid could flow from the workport backwards through the pressure compensating check valve into the pump chamber. As a result, the load could dip, which condition may be referred to as a "backflow" problem.
Another drawback of previous pressure compensating hydraulic control systems is the large number of components. For example the system described in U.S. Pat. No. 5,579,642 provides a chain of shuttle valves which sense the pressure at every powered workport of each valve section. The output pressure of that chain is applied to an isolator valve which connects the control input of the pump to either the pump output or to the tank depending upon the sensed workport pressure. It is desirable to simplify the structure of the pressure compensating hydraulic control system and reduce manufacturing complexity.